Polyacrylate elastomers are well-known synthetic rubbers formed by copolymerization of alkyl acrylates. The polyacrylate elastomers may be polyacrylates that contain only copolymerized alkyl acrylate units, for example copolymerized units of methyl acrylate and butyl acrylate. Alternatively, they may be alkyl acrylate copolymers that contain additional copolymerized monomers, such as ethylene, and cure site monomers such as chlorovinyl ether, monomers that contain carboxyl groups, and/or epoxide containing monomers. The raw polymers, also known as gums or gum rubbers, may be cured with a wide variety of curatives, depending on the cure site monomers. Some acrylate elastomers may be cured with metal soaps such as sodium or potassium stearate, in combination with sulfur, a sulfur donor, a tertiary amine or a quaternary amine salt. Epoxides, isocyanates, and polyols may also be used in certain cases. Polyamines, specifically diamines, are effective curatives for polyacrylates comprising amine-reactive cure sites. Of these curatives, diamines or diamine generators are often preferred because the cured polymers produced exhibit enhanced heat aging resistance. Diamine curable polyacrylates generally contain at least 0.3 mol % of an amine reactive cure site monomer, such as an anhydride, acid-anhydride, half ester anhydride, or epoxide. Some types of polyacrylate elastomers, however, are peroxide curable, even when no cure site monomer is present. Polyacrylate elastomers that have been cured using a diamine cure system generally exhibit heat aging resistance that is superior to that of polyacrylate elastomers that have been cured with other available cure systems. However, diamine cured compounds require a post cure step, for example post cure for four hours at 175° C., to develop optimal properties. Peroxide cured compounds advantageously can be used without a post cure or at most require a short post cure of about 30 minutes at 175° C.
Polyacrylate elastomers comprising only polymerized units of acrylate monomers generally exhibit a poor cure response to peroxide. This is because contiguous polymerized units of acrylate monomers may lead to significant chain scission in the presence of free radicals, so the net increase in crosslink density is low. To enable peroxide curing, the acrylate elastomer must either comprise an unsaturated pendant group which functions as a cure site monomer, or the acrylate elastomer must comprise at least 50 mol % copolymerized units of ethylene. Copolymerized ethylene monomer units act as spacers between polymerized acrylate monomer units to limit β-scission.
Examples of commercially available acrylate elastomers include Vamac® ethylene acrylic elastomers manufactured by E. I. du Pont de Nemours and Company and HyTemp® elastomers, manufactured by Zeon Chemicals L.P.
In view of their excellent oil resistance, polyacrylate elastomers are widely used in the manufacture of automotive parts, such as automotive boots, ignition cable jacketing and hoses.
Resistance to heat aging is a particularly desirable property in rubber parts that are used in under the hood automotive applications, e.g. hoses, gaskets, and seals. Because such parts may be exposed to temperatures in excess of 180° C. for periods of several hours on a regular basis, degradation of physical properties through oxidative embrittlement can occur. In polyacrylate rubbers, a reduction in extensibility and an increase in hardness and modulus of the polyacrylate rubber article often result. Such effects are disclosed for example in Zeon Chemicals L.P., HyTemp® Technical Manual, Rev. 2009-1, p. 59 (2009). Methods to enhance heat age resistance of polyacrylate rubbers have involved attempts to increase the oxidative stability of the polymer by manipulation of the monomer types that comprise the copolymerized units in the polymer backbone including the monomer ratio. In theory, such alterations can provide modified polymer architectures that exhibit increased stability. More effective antioxidants have also been sought. However, there is still a need to improve the high temperature resistance of polyacrylate elastomers.
Although it is known that the presence of fillers can have an adverse effect on high temperature stability of elastomers, the presence of fillers in elastomer formulations (also referred to in the art as elastomer compounds) is generally necessary for reinforcement and development of certain physical properties such as tensile strength and modulus in cured (i.e. crosslinked) compositions and articles comprising the cured compositions. Carbon black is the most widely used filler due to its excellent reinforcement properties and low cost. Other examples of fillers that are commonly used in acrylate elastomers include hydrated alumina, calcium carbonate, barium sulfate, titanium dioxide, magnesium silicate, kaolin clay, and silica. All these fillers adversely affect heat aging of cured acrylate elastomer compositions and articles comprising them.
It has been postulated that fillers accelerate heat aging of polyacrylate elastomers by facilitating the transport of oxygen to the polymer-filler interface. This leads to an increased rate of formation of free radicals at such locations through oxidative reactions. The free radicals that are generated in this manner promote crosslinking reactions, thereby resulting in eventual embrittlement of the elastomer. Reinforcing grades of carbon black such as N330 and N550 are particularly effective at facilitating transport of oxygen because they contain pores that can transport air. However, even non-porous fillers create interfacial regions between the solid filler particles and the elastomer. Few polymer chains reside in such interfacial regions and consequently diffusion of air is enhanced. Thus, exposure of the elastomer to air is generally increased in all filled elastomers compared to unfilled elastomer compositions.
As the reinforcing power of the filler increases, e.g., the ability of the filler to increase Shore A hardness, the tendency of that filler to lower resistance of the acrylate elastomer to the deleterious effects of hot air aging also increases. Such effects are disclosed for a range of carbon black types by Unimatec Chemicals Germany in a publication entitled Noxtite ACM (basic), January 2007, pp. 56-57. It would be desirable to have available an alternative filler that permits the attainment of good elastic properties such as compression set resistance and tensile elongation to break in the cured, filled elastomer and further provides the advantages of filler reinforcement (i.e. high tensile strength, modulus and Shore A hardness), but does not promote oxidative degradation at high temperatures (i.e. 160° C. or greater).
It has now been found that it is possible to produce cured acrylate elastomer compositions of high hardness, strength, and elasticity, that exhibit excellent heat aging resistance through use of polyamide as a filler.
A number of acrylate rubber-polyamide blend compositions have been disclosed in the prior art. For example, it is known to add uncured acrylate elastomers (i.e. gums) to polyamides to form toughened thermoplastic compositions. U.S. Pat. No. 4,174,358 discloses the use of various uncured acrylate elastomers or ethylene based thermoplastics comprising up to 95 mole percent ethylene, such as ethylene/methyl acrylate/monoethyl maleate/ethylenedimethacrylate tetrapolymers, or ionomers of ethylene/methyl acrylate/monoethyl maleate terpolymers as toughening additives for polyamides. The polyamide component in such compositions comprises the continuous matrix and the uncured acrylate elastomer is a minor additive. U.S. Pat. No. 5,070,145 discloses thermoplastic blends of polyamides with ethylene co-polymers comprising units of dicarboxylic acid anhydrides and optionally alkyl(meth)acrylates. U.S. Pat. No. 7,544,757 discloses that blends of ethylene-alkyl acrylate polymers may be blended at levels up to 30% by weight in polyamide to produce toughened polyamide compositions.
Blends of uncured ethylene acrylic elastomers, polyamides and powdered metals are disclosed in Japanese Patent 2001-1191387.
U.S. Pat. No. 3,965,055 discloses vulcanizates prepared from a blend of rubber and 2 wt. % to 10 wt. % of a crystalline fiber-forming thermoplastic, wherein the thermoplastic is dispersed in the rubber component in particles not greater than 0.5 micron in cross section with a length to diameter ratio greater than 2. The high aspect ratio of the thermoplastic particles enables pressureless curing without void formation.
Japanese patent application H10-251452 discloses a dispersion of polyamide particles in a hydrogenated nitrile rubber (HNBR) matrix wherein a compatibilizing polymer that may be an ethylene co-polymer or an acrylate elastomer is also present. The compatibilizing polymer is ionically crosslinked by metal oxide during mixing with the HNBR and polyamide, which prevents the acrylate elastomer from forming the continuous phase. The HNBR component is then peroxide or sulfur cured.
U.S. Pat. No. 6,133,375 discloses blends of functionalized rubbers with thermoplastics in which the thermoplastic is dispersed in the rubber phase. Following addition of a curative for the rubber, the composition is crosslinked to produce a vulcanized article. Examples of functionalized rubbers which are disclosed include acrylic rubbers such as nitrile-butadiene, and hydrogenated nitrile-butadiene, epichlorohydrin, and rubbers on which reactive groups have been grafted such as carboxylated nitrile-butadiene rubber. Thermoplastics that are disclosed include polyether-ester block copolymers, polyurethanes, polyamides, polyamide ethers or ester block copolymers, and mixtures of polyamides and polyolefins. In the latter case, ethylene-alkyl acrylate copolymers comprising grafted or co-polymerized maleic anhydride, glycidyl methacylate, or (meth)acrylic acid may be used to compatibilize the polyamide-polyolefin blend.
U.S. Pat. No. 4,694,042 discloses an elastomeric thermoplastic molding material containing a coherent phase of polyamide and crosslinked elastomeric polyacrylate core shell polymers.
U.S. Pat. No. 4,275,180 discloses blends of thermoplastic polymer with acrylate rubber, the blends being crosslinked or crosslinkable by radiation or peroxide. Fillers may be used in amounts up to 40% by weight of the composition.
U.S. Patent Application 2006/0004147 discloses blends of elastomers, for example an acrylate elastomer, with thermoplastic polymers such as polyamides, in which both polymers are coupled and crosslinked by free radicals, e.g., by electron beam radiation. The compositions may comprise a continuous phase of thermoplastic with dispersed crosslinked elastomer particles, or a continuous crosslinked elastomer phase with dispersed crosslinked particles of what was initially thermoplastic.
U.S. Pat. No. 8,142,316 discloses cured blends of elastomers and thermoplastics for use in power transmission belts. The elastomer may be an ethylene acrylic elastomer, and the thermoplastic may be a polyamide. The ethylene acrylic elastomer is not noted to be amine curable, and only free radical curatives are cited. Free radical curatives are disclosed as curing agents.
It is also known to form dynamically cured thermoplastic compositions having a polyamide matrix continuous phase and a cured acrylate rubber phase that is present in the form of discrete particles. Thermoplastic elastomeric compositions comprising blends of polyamide and ionically crosslinked ethylene acrylic rubber are disclosed in U.S. Pat. No. 4,310,638. U.S. Pat. Nos. 5,591,798 and 5,777,033 disclose thermoplastic elastomer compositions comprising a blend of polyamide resins and covalently-crosslinked acrylate rubber.
U.S. Pat. No. 7,608,216 and U.S. Patent Application Publication 2006/0100368 disclose compositions prepared by admixing an uncured thermoset elastomer, for example an acrylate elastomer, with a thermoplastic polymer or another uncured (gum) elastomer. Techniques such as fractional curing, partial dynamic vulcanization, or the use of high performance reinforcing fillers are disclosed to increase the green strength of the uncured or partially cured compound. The admixed compositions may be subsequently crosslinked with a curing agent for the elastomer component.
Polyacrylate rubber-polyamide blend compositions disclosed in Zeon Chemicals L.P., HyTemp® Technical Manual, Rev. 2009-1, p. 46 (2009) are said to improve the impact strength of plastics. They may also be used to produce thermoplastic elastomers.
It has now been found that when a dispersion of discrete polyamide particles replaces all or a part of the conventional particulate reinforcing agent in a continuous polyacrylate elastomer matrix the resultant compositions, when peroxide cured, exhibit enhanced resistance to embrittlement during heat aging. In addition, such compositions maintain excellent tensile strength, modulus, hardness and elastic properties such as compression set and elongation at break that characterizes compositions containing conventional reinforcing fillers.